1. Field of the Invention
The present invention relates generally to force transducers having a thin film strain gage formed on the surface of a flexural beam, and more particularly, to the construction of a stress reversal beam having an integral limit to prevent excessive travel.
2. Description of the Background Art
Force measurement may be accomplished using a strain gage which converts mechanical motion to an electrical signal. A flexural beam is anchored at one end to a fixed surface and subjected to the force to be measured at the other end. By forming a pattern of resistor elements on the exterior surface of the beam, deformation of the beam as a result of the force can be measured as a function of the change in resistance of the resistor elements as they are stretched or compressed. Commonly, the change in resistance is measured by a Wheatstone bridge circuit, some or all of which may also be formed on the surface of the beam. By mechanically linking the beam to a diaphragm, bellows, or bourdon, pressure measurement can be achieved.
The present invention is concerned with a particular type of strain gage beam referred to as a parallelogram beam and described in U.S. Pat. Nos. 3,341,796 and 4,331,035. The parallelogram beam is designed to create a region of stress reversal intermediate the ends of the beam when the beam is anchored at one end and a force applied near the middle of the beam. In particular, a deflectable element is formed integrally within the beam, which element is connected only to the distal or non-anchored end of the beam. In this way, by applying the force to be measured directly to the deflectable element, an S-shaped or reversing stress is induced in the beam. By placing thin film strain gages on the surface of the beam, particularly at the areas of maximum tension and compression, the flexure of the beam may be measured and related directly to the force applied to the beam.
Although strain gage apparatus employing an integral deflectable element provide more accurate and sensitive force measurement than previously designed beams, the deflectable element is rather fragile and subject to fracture during manufacture and use. Such fragility limits the sensitivity of the gage since a deflectable element and the regions of the beam which undergo stress must be made sufficiently stiff to prevent fracture.
For these reasons, it would be desirable to provide improved parallelogram beam strain gage designs which allow for protection of the integral deflectable element during manufacture and use. In particular, it would be desirable to provide such beams having means for immobilizing the deflectable element during manufacture and for providing variable overload protection for the deflectable element during use.